K-edge x-ray absorption and extended x-ray absorption fine structure (EXAFS) spectroscopies were used to investigate the effects of allosteric modifiers of oxygen affinity, on the coordination geometry of Cu(I) in Panulirus interruptus deoxy-Hc and subunit II of Limulus polyphemus deoxy-Hc. These modifiers included hydrogen ions, L-lactate, and chloride. The coordination geometry of the metal, which is evaluated based on the intensity of the 1s 4pz transition, changes from pseudotetrahedral to trigonal with increasing pH. This structural alteration is correlated with increased oxygen affinity of the proteins at alkaline pH (Bohr effect). EXAFS analysis of both deoxy proteins at high and low pH demonstrate that coordination number and Cu-L bond lengths are not sensitive to pH. Thus, modulation of the coordination geometry of Cu(I) is a feature of the active site governed by the Bohr effect in Hc.